PERFORMANCE EVALUATION OF DOUBLE PASS SOLAR AIR COLLECTOR WITH AND WITHOUT POROUS MATERIAL

Abstract

Solar energy is the most important among renewable energy sources due to its quantitative abundance, its capacity to overcome the energy crisis and environmental threat caused by the continuous use of fossil fuels on a very large scale. The heart of any solar energy related system is solar collector, a type of heat exchanger which transfers solar radiation energy into the heat energy. Solar air collectors are widely used for low to moderate temperature applications like space heating, crop drying, timber seasoning and other industrial applications. Conventional solar air collectors have poor thermal efficiency primarily due to high heat losses and low convective heat transfer coefficient between the absorber plate and flowing air stream, leading to higher absorber plate temperature and greater thermal losses. Attempts have been made to improve the thermal performance of conventional solar air collectors by employing various designs and flow arrangements. The double pass counter flow arrangement with porous material in the second air passage is one of the important and attractive design improvement that has been proposed to achieve the objective of improved thermal performance. The major reason of interest in porous packing includes its high effective heat transfer area per unit volume of packed duct resulting in high heat transfer capability. Also solar radiations are progressively absorbed by the layers of wire mesh and the remaining radiations are absorber by the absorber plate. Thus not only the absorber plate but the matrix material also works as the absorber of solar radiation. Therefore, the heat energy due to absorption of solar radiation is now distributed throughout the packed material and the absorber plate and dissipated now more effectively to the flowing fluid due to very large surface area being in contact with the flowing fluid in a packed collector as compared to a plane collector where in only the absorber plate surface is in contact with the flowing fluid. The packing material also provides an increased number of sharp corners in the flow passage which enhance the level of turbulence, causing proper mixing and preventing the build-up of slow moving layer of air. Therefore the heat transfer rate from the porous material to the flowing air is much more resulting in its improved thermal performance. From the review of literature, it has been observed that although considerable amount of research work has been carried out on heat transfer and pressured drop in packed beds having packing elements of different material and geometrical parameters; yet it has been found that the situation for which these studies have been performed and correlations developed are quite different from those encountered in case of solar air collectors. Most of the reported work and correlations are applicable for flow through tubes and ducts, where the energy distribution in the bed is fairly uniform. The solar air collector with packing material as wire mesh screen matrices has highly non-uniform energy distribution and available correlations can not be used for prediction of heat transfer and pressure drop characteristics. It is also noted that there are many studies on the single pass solar air collector, but the outdoor experimental studies on double pass counter flow flat-plate solar air collector where in air flow is parallel to the plane of the wire mesh layers and the solar radiations are in a direction perpendicular to this plane has not been reported so far. In view of the above, the present investigation has been undertaken with the following distinct objectives: (i). To design and fabricate the experimental set up for double pass counter flow solar air collector with and without porous matrix in the second pass to collect outdoor experimental data on heat transfer and pressure drop. If; (ii). To investigate the thermal performance of a double pass solar air collector with porous matrix in the second pass and compare its performance with that of a plain double pass solar air collector i.e. without matrix in the second pass. (iii). To study the effect of various system and operating parameters such as mass velocity of air, intensity of solar radiation, inlet air temperature, geometry of matrix and porosity on the thermal performance of double pass solar air collector with and without porous absorber matrix. (iv). To carry out an experimental investigation to determine extinction coefficient and volumetric heat transfer coefficient of matrix absorber and to develop correlations for packed duct volumetric heat transfer coefficient, friction factor and extinction coefficient on the basis of experimental data in terms of operating and geometrical parameters of the packed collector. (v). To develop mathematical model based on volumetric heat transfer coefficient considering a separate energy balance for solid matrix material and flowing fluid to predict the thermal performance of double pass counter flow solar air collector with matrix material and also to compare its performance with double pass counter flow solar air collector without matrix and other conventional solar air collectors. (vi). To carry out the thermohydraulic or effective performance analysis of double pass solar air collector with and without porous material. The experimental set up has been designed, fabricated and tested outdoor in accordance with the ASHRAE standard 93-77 recommendations for the data collection on heat transfer and pressure drop. Two types of wire mesh covering twelve different porosities have been used as a porous material. The performance of double pass counter flow solar air collectors have been studied for two conditions of inlet air temperature; one is near the ambient air temperature and the other when inlet air vii temperature is maintained approximately 10 °C higher than the ambient temperature. Experimental data were collected for temperature at the inlet and exit of the double pass collector, temperature of each wire mesh layers, absorber plate temperature, the pressure drop across collectors, mass flow rate, intensity of solar radiation, ambient air temperature and wind velocity. The data for a plain double pass counter flow solar air collector under similar operating conditions were collected to compare its performance with that of double pass counter flow solar air collector with matrix material. Various geometrical and operational parameters considered in the present investigation are given below. Mass velocity, Go : 0.0121 to 0.0544 Kg/m2-s (based on unit collector area) Mass velocity, Go' : 1.231 to 5.461 Kg/m2-s (based on cross sectional area of plain collector) Mass velocity, Gop : 1.233 to 6.122 Kg/m2-s (based on cross sectional area of packed collector) Number of wire mesh layers, n : 1 to 7 (Matrix W1) and 1 to 5 (Matrix W2) Wire diameter, 4, : 0.42 mm (Matrix W1) and 1.05 (Matrix W2) Longitudinal and transverse pitch,: pi = pt =2.5 mm (for both matrix W1 and W2) Porosity, p : 0.892 to 0.994 Intensity of solar radiation, I : 500-1050 W/m2 Inlet air temperature, Tfi : and Tfi --z,'Ta+10 °C Plain duct Reynolds number, Re : 2500 to 11215 (based on Go', and De,) Packed duct Reynolds number, Rep : 610 -19750(based on Gop', and Dep,) viii The possible uncertainties in the experimental results were examined. The uncertainties of the values of major parameters determined for all set of test runs showed maximum uncertainties as: Mass velocity ± 2.55 %, thermal efficiency ± 2.70 %, pressure drop across collector ± 15.76 %, effective efficiency ± 15.93 % and volumetric heat transfer coefficient ± 3.48 %. The following correlations have been developed for volumetric heat transfer coefficient, friction factor and extinction coefficient of porous material. k = 0.45(k-c)(Rep )0.62 (p)-4.56() -0.21 (pol / 3 D ep fp = 73.25(Re -0.67 (p)-2.91 (Ey) -0.52 In(-2-)= e cos ei The comparison of experimental and predicted values froni the developed correlations reveals a satisfactory prediction of volumetric heat transfer coefficient, friction factor and extinction coefficient for packed collector. It is observed that the thermal efficiency of double pass counter flow solar air collector with matrix material in the second air pass is significantly higher compared to plain collector. It is also observed that the thermal performance of double pass solar air collector with porous matrix is greatly influenced by the porosity of second air pass i.e. number of wire mesh layers in the second passage. It is noted that the thermal efficiency of packed collector increases as the number of wire mesh layers increases or with decreases in porosity. This may be due to the fact that decrease in porosity increases the effective heat transfer area per unit volume of packed duct. Therefore, the volumetric heat transfer coefficient between the matrix and flowing air increases which results in higher thermal efficiency. Also, decrease in porosity reduces the flow channeling i.e. increase in tortuousness of the air flowing through the packed passage iv resulting in improved thermal performance. The packing material also provides an increased number of sharp corners in the flow passage which enhance the level of turbulence, causing proper mixing and preventing the build-up of slow moving layer of air. Therefore the heat transfer rate from the porous material to the flowing air is more resulting in improved thermal performance. The another reason for enhancement of its thermal performance is due to the fact that in case of packed collector, solar radiations are progressively absorbed by the layers of wire mesh and the remaining radiations are absorber by the absorber plate. Thus not only the absorber plate but the matrix material also works as the absorber of solar radiation. The pressure drop in both the collectors has been measured and the thermohydraulic or effective performance is presented. The comparison of thermal efficiency and effective thermal efficiency reveals that at lower mass velocity and higher porosity both the efficiencies are nearly the same, whereas at higher mass velocity and lower porosity there is a substantial difference between them; the effective efficiency being considerably lower. A mathematical model is developed based on volumetric heat transfer coefficient. The values of thermal efficiency predicted by the mathematical model are compared with the experimental values. The absolute average deviation between these values are found to be 9.0 % and 11.5 % for double pass counter flow solar air collector with and without matrix material respectively.